A thermal fuse device, a current fuse device, a polymer PTC device, or the like is used as a protection device which interrupts an excessive current when the excessive current flows during charging or discharging of a cylindrical lithium-ion secondary battery. In particular, the polymer PTC device is useful in that since it can be positioned by incorporating into a sealing plate of the secondary battery, a battery pack which is particularly composed of many of the secondary batteries becomes to be compact.
However, a large amount of current (for example, a current of 10 A) cannot be continuously flowed through for example a commercially available annular PTC device. Furthermore, the PTC device has a resettability to be in a low resistance state when an abnormality is removed and its temperature is decreased, but may cause a problem depending on its application. For example, in case wherein the PTC devices are used in a cylindrical lithium-ion secondary battery cell which is used in multi-parallel, a shorting cell in which the PTC device continues to produce heat and there is a possibility that the cell bursts as long as such cell is not removed.
In view of the above described problem, it was proposed, for example, that a spacer is used in place of the PTC device inside the sealing plate in the cylindrical lithium-ion secondary battery cell (see Non-Patent Reference 1 below). However, when the spacer is used, there is a problem that a protection from the excessive current cannot be ensured.
Matsushita Technical Journal, Vol. 52, No. 4, August 2006, pp. 31-35, is a prior reference.